Memory devices are frequently provided as internal, semiconductor, integrated circuits and/or external removable devices in computers or other electronic devices. There are many different types of memory, including volatile and non-volatile memory. Volatile memory, including random-access memory (RAM), static random access memory (SRAM), dynamic random access memory (DRAM), and synchronous dynamic random access memory (SDRAM), among others, may require a source of applied power to maintain its data. Non-volatile memory, by contrast, can retain its stored data even when not externally powered. Non-volatile memory is available in a wide variety of technologies, including flash memory (e.g., NAND and NOR) phase change memory (PCM), resistive random access memory (RRAM), and magnetic random access memory (MRAM), among others.
Flash memory devices can include an array of memory cells that each store data in a charge storage structure, such as a conductive floating gate or a dielectric charge trap. Flash memory devices frequently use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption during use. Memory cells in a flash memory device can be programmed to a target state to represent information. For instance, electric charge can be placed on, or removed from, the charge storage structure (e.g., the charge trap or the floating gate) of a memory cell to program the cell to a particular data state. The data state can subsequently be read back from the memory cell by measuring a resistance of a circuit including the cell to determine the amount of charge on the charge storage structure.
Given the high density of flash memory cells, operations (e.g., reads, writes, erasures) on one memory cell can impact the charge stored on adjacent or nearby cells. One such effect is known as “read disturb,” in which a read operation performed on memory cells connected to one word line can change the amount of charge stored in memory cells on other word lines (e.g., the adjacent or nearby word lines) in the same memory block.
One approach to address this problem involves tracking the number of read operations that have occurred in a memory block (spanning multiple word lines) so that the data therein can be pre-emptively relocated to a different memory block before the number of read operations reaches levels that can cause data loss. This approach requires characterizing a number of read operations which can be safely performed on a memory block. This characterization poses a challenge, however, as the number of read operations that can be safely performed on a memory block depends upon the distribution of the read operations within the memory block. For example, in a memory block in which the read operations are evenly distributed across word lines, the number of read operations that could safely be performed would be fairly high, whereas in a memory block in which the read operations are concentrated on one or a few word lines, the number of read operations that could safely be performed would be fairly low. Choosing a single threshold value of read operations for every memory block in a device, therefore, will provide poor performance where some memory blocks experience more evenly distributed read disturb effects and others experience more localized read disturb effects on just a few word lines. Accordingly, a way to more efficiently address the localized effects of read disturb is required.